- Email: [email protected]
Leachate treatment in landfills is a significant N2O source
Science of the Total Environment 596–597 (2017) 18–25
Contents lists available at ScienceDirect
Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv
Leachate treatment in landfills is a significant N2O source Xiaojun Wang a, Mingsheng Jia a, Chengliang Zhang a, Shaohua Chen a,⁎, Zucong Cai b,⁎⁎ a b
CAS Key Laboratory of Urban Pollutant Conversion, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, China School of Geography Sciences, Nanjing Normal University, Nanjing 210023, China
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
• N2O emissions from leachate system were intensive, while often ignored. • The static chamber method was adopted to measure GHG emissions in landfills. • A first set of GHG emissions from reservoirs and leachate in landfills was collected. • High N2O EF of 8.9–11.9% was achieved in leachate treatment systems. • N2O should be included in the inventory of GHG emissions from landfills.
a r t i c l e
i n f o
Article history: Received 9 February 2017 Received in revised form 1 April 2017 Accepted 5 April 2017 Available online xxxx Editor: Jay Gan Keywords: N2O emissions Leachate treatment systems Landfill reservoirs GHG inventory IPCC
a b s t r a c t The importance of methane (CH4) emissions from landfills has been extensively documented, while the nitrous oxide (N2O) emissions from landfills are considered negligible. In this study, three landfills were selected to measure CH4 and N2O emissions using the static chamber method. Dongbu (DB) and Dongfu (DF) landfills, both located in Xiamen city, Fujian Province, were classified as sanitary. The former started to receive solid waste from Xiamen city in 2009, and the latter was closed in 2009. Nanjing (NJ) landfill, located in Nanjing county, Fujian Province, was classified as managed. Results showed that for the landfill reservoirs, CH4 emissions were significant, while N2O emissions occurred mainly in operating areas (on average, 16.3 and 19.0 mg N2O m−2 h−1 for DB and NJ landfills, respectively) and made a negligible contribution to the total greenhouse gas emissions in term of CO2 equivalent. However, significant N2O emissions were observed in the leachate treatment systems of sanitary landfills and contributed 72.8% and 45.6% of total emissions in term of CO2 equivalent in DB and DF landfills, respectively. The N2O emission factor (EF) of the leachate treatment systems was in the range of 8.9–11.9% of the removed nitrogen. The total N2O emissions from the leachate treatment systems of landfills in Xiamen city were estimated to be as high as 8.55 g N2O-N capita−1 yr−1. These results indicated that N2O emissions from leachate treatment systems of sanitary landfills were not negligible and should be included in national and/or local inventories of greenhouse gas emissions. © 2017 Elsevier B.V. All rights reserved.
1. Introduction ⁎ Correspondence to: S. Chen, CAS Key Laboratory of Urban Pollutant Conversion, Institute of Urban Environment, Chinese Academy of Sciences, 1799 Jimei Road, Xiamen 361021, China. ⁎⁎ Corresponding author. E-mail addresses: [email protected] (X. Wang), [email protected] (M. Jia), [email protected] (C. Zhang), [email protected] (S. Chen), [email protected] (Z. Cai).
http://dx.doi.org/10.1016/j.scitotenv.2017.04.029 0048-9697/© 2017 Elsevier B.V. All rights reserved.
Waste disposal sites (solid waste and wastewater) are recognized as important sources of global anthropogenic greenhouse gas (GHG) and contributed approximately 3–4% of the annual budget in term of their CO2 equivalent (IPCC, 2007). In China, the GHG emissions from waste
X. Wang et al. / Science of the Total Environment 596–597 (2017) 18–25
treatment were estimated to account for 1.5% of the total national anthropogenic GHG emissions in 2005 (National Development and Reform Committee of China, 2013). Landfill is one of the main disposal methods of solid wastes around the world, and CH4 emissions from landfills are taken into account for national inventories of GHG emissions. In the United States, the CH4 emission from landfills was estimated to account for approximately 2.4% of the national anthropogenic GHG emissions and up to 21.6% of the total anthropogenic CH4 emissions between 1990 and 2008 (US EPA, 2009). The CH4 emissions from solid waste disposal are predicted to continuously increase as economic development advances and population rise (Monni et al., 2006). On the other hand, due to the strict anaerobic conditions inside landfills, nitrification of ammonium, which takes place under aerobic conditions (Bremner & Blackmer, 1978), is inhibited. And denitrification of nitrate, which occurs under anaerobic conditions (Wrage et al., 2001), is processed completely. Thus, as a byproduct of nitrification and an intermediate product of denitrification processes (Maag & Vinther, 1996; Sutka et al., 2006), N2O emissions from landfills are assumed to be negligible and are not taken into account in the 2007 IPCC Guidelines for National Greenhouse Gas Inventories (IPCC, 2007). However, the assumption that N2O emissions from landfills are negligible is not accurate. In situ measurements of N2O emissions from landfills showed that their N2O fluxes were one or more orders of magnitudes higher than those from forestlands, grasslands and farmlands (Cai, 2012; Rinne et al., 2005). There are several possible hotspots of N2O production and emission in landfills. One of the possible hotspots is the operating area where both aerobic and anaerobic conditions co-exist. Here, the produced N2O can be directly emitted into the atmosphere because no coverage prevents its transport. It has been reported that in a case-study of landfills, the peak N2O flux was up to 200 g CO2-eq m−2 h−1 (Harborth et al., 2013). Another possible hotspot is the landfill leachate treatment system. In China, landfills are required to establish an integrated leachate treatment system, and the effluent of leachate must reach a standard before discharged into water bodies in accordance with the Chinese National Standard (GB16889-2008). Nitrogen in leachates is generally removed by processes of alternative biological nitrification and denitrification (Qiu et al., 2010). On account of high contents of ammonium and organic matter (Bae et al., 1997; Renou et al., 2008), the N removal processes of landfill leachates are assumed to be active in N2O emission (Desloover et al., 2012). However, GHG emissions of the wastewater section consist only of municipal and industrial wastewater collection and treatment, not landfill leachates (IPCC, 2007). In the present study, comprehensive measurements of CH4 and N2O emissions from landfill reservoirs and leachate treatments were conducted in three landfills differing in classification and status. The objectives of the study were to identify the hotspots of CH4 and N2O emissions and to evaluate the contribution of N2O emission to total CH4 and N2O emissions from landfills in term of global warming potential. 2. Materials and methods 2.1. Landfill description Landfills are classified into three grades according to the environmental protection measures in China, i.e., simple landfills, managed landfills and sanitary landfills (Ma & Gao, 2011). For simple landfills, there are no environmental protection measures preventing leachates from infiltrating underground water, or landfill gases (LFGs) from being emitted into the atmosphere. Simple landfills are mainly distributed throughout countryside. Environmental protection measures are available for managed landfills. These measures prevent leachates from infiltrating underground water and require that leachates are treated, but landfills are not covered well by plastic film and/or soil, and the facilities for LFGs collection are not available. Managed landfills
19
are primarily distributed throughout the suburbs of county cities. Sanitary landfills have perfect environmental protection measures and facilities for preventing leachates from infiltrating underground water and halting LFG emission into the atmosphere by treating leachates and collecting and utilizing LFGs. Except for in the operating area, the reservoirs of landfills are well covered by high density polyethylene (HDPE) film. Sanitary landfills are mainly distributed in the suburbs of middle and large cities. For this study, three landfills with different management levels in Fujian Province were selected. CH4 and N2O emissions were sampled at these landfills over a one-year timeframe. (1) Dongbu Landfill (DB) in Xiamen City: classified as an active sanitary landfill, DB is designed for waste loading of 7.29 million tons and a 30-year expected operating period. From 2009 to now, DB landfill received municipal solid waste at an average load of 2100 tons per day. The reservoirs occupied 25.88 ha. Except for the operating area, landfill reservoirs are all covered by HDPE (Fig. 1a). The LFG collection system is established for CH4 recovery. The compositions of solid waste received by DB landfill are characterized and shown in Table 1. (2) Dongfu Landfill (DF) in Xiamen City: DF was also a sanitary landfill, but closed in 2009. The whole reservoir of landfill, occupying 18 ha, was covered by HDPE film, followed by a soil layer of approximately 0.5 m in depth with grass restoration (Fig. 1b). The landfilled waste reached a total of 3.8 million tons. The LFG collection system was also installed and collected LFG was used for electricity power generation. (3) Nanjing Landfill (NJ) in Nanjing County: NJ is an active managed landfill (Fig. 1c) and mainly receives the domestic solid waste from Nanjing County from 2008 to present, with a daily average load of 70 tons. The area of reservoir was 1.15 ha. 2.2. Leachate treatment systems in landfills Landfill leachates were first stored in leachate ponds and then treated by biological processes. A/O-ultrafiltration-nanofiltration, A/Oultrafiltration and oxidation ditches were applied to treat landfill leachates in DB, DF and NJ landfills, respectively (Fig. 2). The treating capacity and water characteristics of the influent and effluent were presented in Table 2. The N removal rates were, on average, 72.9% and 79.7% in DB and DF leachates, respectively. Since the oxidation ditch in NJ was poorly managed, the N removal rate was extremely low (14.5%). 2.3. Gas sampling For DB landfill reservoirs, six plots were set up in the operating area and six plots were set up in HDPE film covered areas to measure CH4 and N2O fluxes. CH4 and N2O fluxes were measured using the static chamber method in a two week intervals from April 2012 to April 2013. The CH4 and N2O emissions from DF landfill and NJ landfill reservoirs were measured by the static chamber method after creating grid cells of 40 × 40 m (Fig. 3a and b), following the ‘Guidance on monitoring landfill gas surface emissions’ proposed by the UK Environmental Agency (UK Environment Agency, 2010). For DF landfill reservoirs, CH4 and N2O fluxes from each cell were measured twice in January and July 2014, respectively. For NJ landfill reservoirs, CH4 and N2O fluxes from each cell were measured six times over a one-year timeframe (February 2015–March 2016) at an interval of two months. For the leachate storage and treatment systems, CH4 and N2O fluxes from each compartment were measured and gas sampling plots were shown in Fig. 2. Two methods, floating static chamber and gas flow rate, were employed to measure CH4 and N2O fluxes from a nonaerated surface and an agitated surface, respectively. The volumetric flow rate was obtained by the recorded flow velocity (4500 Pocket Weather Tracker, Kestrel) multiplying by the corresponding area of the off-gas vent. The mass flow rate was calculated from the measured
20
X. Wang et al. / Science of the Total Environment 596–597 (2017) 18–25
Fig. 1. A panorama of the case-study landfills in the suburb of Xiamen (a) the map of sampling sites; (b) DB: an active sanitary landfill; (c) DF: a closed sanitary landfill; (d) NJ: an active managed landfill.
gas concentration and the volumetric flow rate of the off-gas originating from the covered reactor zones. The procedure for sampling was described in detail by Wang et al. (2014). Three sampling rounds were conducted during April/May of 2013 for DB and DF leachate treatment systems. For NJ landfill leachate treatment system, the measurements of CH4 and N2O fluxes were undertaken every two months using the floating static chamber, synchronized with the measurements in landfill reservoirs.
landfill reservoirs was combined with spatial interpolation (Inverse Distance Weighting, IDW) based on geostatistics (ArcGIS 9.3, ESRI, USA). To compare the contributions of CH4 and N2O emissions to global warming potential, the emissions were expressed in CO2 equivalents (CO2-eq), in which CH4 and N2O had 25 and 298 times the global warming potential than that of CO2, respectively, at a time scale of 100 yr (IPCC, 2013). The N2O emission factor (EF) of removed N in landfill leachates was used to evaluate the capacity of N2O emission from the leachate system (Eq. (2)).
2.4. Analyses and calculation To measure CH4 and N2O fluxes by the static chamber method, five sequential gas samples were withdrawn from chamber headspace by syringe at 10-min intervals for landfill reservoirs and 5-min intervals for leachate treatment systems. Off-gas samples were also collected to determine CH4 and N2O fluxes from aerated bioreactors. All gas samples were analyzed by using a gas chromatograph (GC, Agilent 7890A, Palo Alto, CA), fitted with a flame ionization detector (FID) for CH4 detection and an electron capture detector (ECD) for N2O detection. The fluxes were calculated based on the change in concentrations over time (dC/ dt), chamber height and temperature correction (Eq. (1)) (Song et al., 2006). F¼
dC M 273 h dt V0 T þ 273
ð1Þ
where F is the flux (mg CH4 m−2 h−1 or mg N2O m−2 h−1); dC/dt is the change in CH4 or N2O concentration over time; M is the mole mass of CH4 or N2O; V0 is the mole volume of the target gas under standard conditions; T is the air temperature; h is the headspace height of the chamber. The annual CH4 and N2O emissions were calculated using the average fluxes multiplied by the time of year in hours and the size of the corresponding study sections. The average fluxes were assumed to be representative of annual emissions for each section. The calculation of annual emissions from the full-scale field monitoring in DF and NJ
EF ¼
E MN;Inf −MN;Eff
ð2Þ
where E is the N2O emission of whole treatment system, MN,Inf is the total amount of nitrogen influent (kg N d−1) and MN,Eff is the total amount of nitrogen effluent (kg N d−1). The denominator in the equation was the nitrogen amount removed by the treatment system. An elemental analyzer (Vario MAX, Elementar, Germany) was used to determine carbon and nitrogen contents in solid waste. For leachate, − − the ammonium (NH+ 4 ), nitrite (NO2 ), nitrate (NO3 ) and total nitrogen (TN) concentrations were analyzed by using spectrophotometry described in standard methods (Chinese NEPA, 2002). The colorimetric method was adopted to determine the chemical oxygen demand (COD) of landfill leachate with COD analyzer (5B-1, Lianhua Technology, China). 3. Results In the study, CH4 and N2O sources were divided into landfill reservoir and leachate categories. The former was further divided into operating area and covered area (except for the closed DF landfill, which had no operating area) and the latter was further divided into leachate storage pond and treatment system. The CH4 and N2O emissions from each compartment of DB, DF and NJ were presented as follows.
Table 1 Physical and chemical characteristics of municipal solid waste disposed in DB landfill. Compositiona %
MSW a
Food waste
Papers
Plastics
Textiles
Woods
Glasses
Metals
Dust
51.5
9.3
15.0
5.1
2
2.9
0.7
13.5
Provided by Environmental Sanitation Management Department of Xiamen City.
Moisture %
C%
N%
Food waste C%
N%
48.4 ± 2.5
45.3 ± 4.4
1.12 ± 0.04
49.5 ± 2.7
2.57 ± 0.26
X. Wang et al. / Science of the Total Environment 596–597 (2017) 18–25
21
Fig. 2. Sampling sections of leachate treatment processes at three landfills. (a) DB for the young leachate; (b) DF for the aged leachate; (c) NJ for the young leachate.
3.1. CH4 and N2O emissions from DB landfill HDPE film could prevent the CH4 and N2O emissions effectively in DB landfill. The CH4 fluxes from the HDPE film covered surface ranged between 0.179 and 70.3 mg CH 4 m − 2 h − 1 with an average of 16.3 mg CH4 m− 2 h− 1. N2O fluxes ranged between 0 and 156 μg N2O m−2 h−1 with an average of 33 μg N2O m−2 h−1. N2O emissions from the landfill reservoir mainly occurred in the operating area. The mean N2O flux reached 16.4 mg N2O m−2 h− 1 in the operating area, which contributed 97% of the total N2O emission from the reservoir. The CH4 and N2O emissions from the DB landfill reservoir were 96.0 t CH4 yr−1 and 2.52 t N2O yr−1, respectively (Table 3). N2O and CH4 contributed 23.8% and 76.2% of the total emissions from the reservoir in CO2-eq, respectively. A substantial amount of CH4 emission was detected from the leachate storage pond compared to the landfill reservoir. The landfill discharged leachate at approximately 470 m3 per day (Table 2) and the CH4 flux from the storage pond was as high as 18.0 g CH4 m−2 h−1 on average. The accumulated CH4 emission from the storage pond was as large as 4360 t CH4 yr− 1 (Table 3). A small amount of CH4 emission occurred in leachate treatment systems as well, particularly from the denitrifying tank. The annual CH4 emission from the leachate treatment system was 2.26 t CH4 yr−1. In sharp contrast with CH4 emission from the leachate, N2O emissions mainly occurred in the leachate treatment system (34.42 t N2O yr−1) and the emission from the leachate storage pond was relatively small (87.8 kg N2O yr−1). In the leachate system, nitrifying processes were the dominant N2O source, accounting for 74.7% and 16.9% of the total
N2O emission for nitrifying tanks 1 and 2, respectively. The denitrifying tank contributed only 8.2% of the total N2O emission from the leachate treatment system (Table 3). Considering that the CH4 emitted from leachate storage pond was used for electricity generation, it was not included as an emission to the atmosphere. Therefore, the total CH4 and N2O emissions to the atmosphere from the whole DB landfill was 13.5 Gg CO2-eq yr−1, in which 18.2% were contributed by CH4 and 81.8% by N2O (Fig. 4a). Of these emissions, 23.4% were from the reservoir and 76.6% were from the leachate (Fig. 4b). 3.2. CH4 and N2O emissions from DF landfill DF landfill was closed in 2009 and whole reservoir was covered by HDPE film, followed by a soil layer and grasses that were grown on the covered soils. The CH4 and N2O emissions at the 98 sampling sites of the grid cells were detectable. Both CH4 and N2O fluxes had high spatial variability with the range of coefficient of variation (CV) from 268% to 421%. The averaged fluxes of CH4 and N2O were 139 mg CH4 m−2 h−1 and 189 μg N2O m−2 h−1, respectively (Table 4). There were no significant differences between the fluxes measured in summer and winter in terms of CH4 and N2O (P N 0.05). The annual CH4 and N2O emissions from DF landfill reservoir were 219.6 t CH4 yr−1 and 0.3 t N2O yr−1 (Table 3), respectively, and the contribution of N2O to the total emission was only 1.6% in terms of CO2-eq. DF landfill discharged leachate at approximately 250 m3 d− 1 (Table 2). The CH4 fluxes from the leachate storage pond were much lower than those of DB landfill. The accumulated CH4 emission in DF
674.1 ± 277.6 970.1 ± 408.8 2120.0 ± 270.1
leachate storage pond was only 0.05 t CH4 yr−1 (Table 3). The CH4 from the leachate storage pond of DF landfill was not collected, but emitted to the atmosphere directly. Thus, the emission contributed to the total emission from the DF landfill. Like DB landfill, the leachate treatment system of DF landfill was also a big source of N2O, mainly from nitrifying tanks 1 and 2. The total N2O emission from the leachate treatment system was 14.94 t N2O yr−1, of which 68.1% was contributed by nitrifying tank 1 and 20.2% by nitrifying tank 2 (Table 3). The total CH4 and N2O emissions to the atmosphere from DF landfill were 10.0 Gg CO2-eq yr−1. The contribution of CH4 (54.8%) was slightly more than that of N2O (45.2%) to the total emission (Fig. 4a). The landfill reservoir and leachate contributed 55.6% and 44.4%, respectively (Fig. 4b).
30.7 ± 29.8 2068.8 ± 128.2 2127.2 ± 277.8 1491.4 ± 107.2 Fresh Aged Fresh 470 250 15 DB DF NJ
3.3. CH4 and N2O emissions from NJ landfill
“-” means non-detectable.
COD TN
663.1 ± 129.6 438.1 ± 112.9 1618.4 ± 110.0 608.5 ± 292.3 384.7 ± 146.7 168.2 ± 103.3 12.0 ± 11.4 10.9 ± 5.4 1173.3 ± 44.2
COD
8295 ± 1090 2150 ± 77 2776.5 ± 125.2
TN
Fig. 3. The designed sampling sites of full-scale field monitoring campaigns in (a) closed DF sanitary landfill and (b) active NJ county landfill.
16.9 ± 1.7 14.1 ± 11.3 276.7 ± 19.8
2448.5 ± 265.9 2160.5 ± 222.0 1875.3 ± 43.0
1.5 ± 2.1 6.8 ± 8.7 217.2 ± 104.1
NO− 3 -N NO− 2 -N NH+ 4 -N
Effluent (mg L−1)
NO− 3 -N NO− 2 -N NH+ 4 -N
Influent (mg L−1)
Leachate status Loading (m3 d−1)
Table 2 Water characteristics of leachate treatment systems in three landfills.
72.9% 79.7% 14.5%
X. Wang et al. / Science of the Total Environment 596–597 (2017) 18–25 TN removal rate
22
The reservoir of NJ landfill was covered by a soil layer, except for the operating area. The averaged CH4 fluxes over each measurement campaign ranged from 405 to 1914 mg CH4 m−2 h−1 with an overall average of 1089 ± 517 mg CH4 m− 2 h− 1. The corresponding N2O fluxes ranged from 1.33 to 6.57 mg N2O m−2 h−1 with an overall average of 2.95 ± 1.43 mg N2O m− 2 h− 1. The annual CH4 and N2O emissions from the reservoir were 122 t CH4 yr−1 and 0.34 t N2O yr−1, respectively. N2O contributed only 3.2% of the total emission in term of CO2-eq. The emissions from leachate were 0.16 t CH4 yr−1 and 0.15 t N2O yr− 1 (Table 3). The CH4 produced in the leachate storage pond was not collected and used in NJ landfill. Therefore, the total
X. Wang et al. / Science of the Total Environment 596–597 (2017) 18–25 Table 3 Annual CH4 and N2O emissions from landfill reservoirs and individual bioreactor compartments of three leachate treatment systems. Source
Reservoir Leachate Storage pond Treatment system Denitrifying tank Nitrifying tank-1 Nitrifying tank-2 Oxidation ditch Secondary sedimentation tank Sludge thickening tank Nanofiltration concentrate tank
CH4 (t CH4 yr−1)
N2O (t N2O yr−1)
DB
DF
NJ
DB
DF
NJ
96.0 2.26a 4360 2.26 1.37 0.55 0.09 – – 0.14 0.11
219.6 0.33 0.05 0.28 0.05 0.08 0.07 – – 0.08 –
122 0.16 0.14 0.02 – – – 0.02 b0.01 – –
2.52 34.50 0.09 34.41 2.83 25.72 5.82 – – 0.04 b0.01
0.3 14.94 b0.01 14.94 1.74 10.17 3.02 – – 0.01 –
0.34 0.15 0.01 0.14 – – – 0.14 b0.01 – –
a The CH4 emitted from the leachate storage pond was used for electricity generation, so it was not included into the GHG inventory of DB landfill.
emission to the atmosphere, including the CH4 emissions from leachate, was 3.2 Gg CO2-eq yr−1. Unlike the DB and DF landfills, the contribution of leachate to the total emission was only 1.5% (Fig. 4a) and N2O emissions from the reservoir and leachate accounted for only 4.6% of the total (Fig. 4b).
23
4. Discussions 4.1. Negligible N2O emission from landfill reservoirs N2O emissions from landfills are not included in the 2007 IPCC Guidelines for National Inventory of Greenhouse Gas Emissions because the emissions are thought to be negligible. This is true if only the emissions from landfill reservoirs are considered. HDPE film coverage of sanitary landfills effectively prevented N2O emissions from landfill reservoirs. The mean N2O flux from the HDPE film covered areas of DB landfill reservoir (0.033 mg N2O m− 2 h− 1) was much less than that from the operating area (16.3 mg N2O m−2 h−1) and the overall average from the NJ landfill reservoir (2.95 mg N2O m−2 h−1). The reservoir of DF landfill was covered by HDPE film, followed by a soil layer with grass growth (Fig. 1b). The N2O fluxes from the reservoir of DF landfill (averagely 188.6 μg N2O m−2 h−1, Table 4) were substantially larger than that from the HDPE surface of DB landfill, and slightly larger than those from subtropical terrestrial ecosystems (Hu et al., 2015). This indicates that the emitted N2O from the reservoir of DF landfill is likely produced mainly in the covered soil layer, rather than in the deposited wastes. The N2O emissions accounted for only 23.8%, 1.6% and 3.2% of the total CO2-eq emissions from DB, DF and NJ landfill reservoirs, respectively. For DB landfill reservoir, 97% of N2O emissions occurred in the operating area. 4.2. Significant N2O emissions from leachate treatment of sanitary landfills Landfill leachate was characterized by extremely high NH+ 4 concentrations, which were N2000 mg N L−1 in DB and DF landfills on average (Table 2). High N content in landfill leachate was attributed to the large proportion (28–60%) of food in municipal solid wastes in China. Compared with other components of solid waste, food waste had a higher N content leading to a low C/N ratio (Zhou et al., 2014). The large amount (55.1%) of food waste made the overall N content in Xiamen municipal solid wastes relatively high (Table 1). Several treatment processes have been developed to remove N in landfill leachates. A/Oultrafiltration-nanofiltration and A/O-ultrafiltration were applied to treat landfill leachates in DB and DF landfills, respectively (Fig. 2). The N removal efficiencies were 72.9% in DB and 79.7% in DF. These are comparable to those of lab-scale and full-scale treatment systems reported in the literatures (Lin et al., 2008; Wu et al., 2015), but N contents in effluents were still very high (Table 2). NH+ 4 content in the leachates of NJ landfill (1491 mg N L−1) was lower than those in DB and DF landfills. An oxidation ditch was used to remove N in the leachates in NJ landfill, but the efficiency was very low (14.5%). Further, the total N content in the effluent was even higher than those in DB and DF landfills (Table 2), indicating that the treatment system had to be improved to minimize the environmental impacts of discharged landfill leachates. Large N2O emissions from the leachate treatment systems were observed in DB and DF landfills. The N2O emissions from the leachate treatment system accounted for 76.2% and 44.4% of the summed emissions from landfill reservoirs and leachate treatment systems in term of CO 2-eq for DB and DF landfills, respectively. Taking the Table 4 Summary of CH4 and N2O fluxes from DF landfill reservoir, which was measured by following the ‘Guidance on monitoring landfill gas surface emissions’ proposed by the UK Environment Agency (2010). Target gas
Sampling time
Mean
Min
Max
SD
CV (%)
Percentage of negative value
N2 O
2014-01 2014-07 2014-01 2014-07
172.9 204.3 94.9 113.0
−47.1 −47.1 −6.3 −14.6
4211 6789 1555 1823
628.6 848.6 254.2 325.0
377.9 420.5 267.8 288.7
9.2 8.2 21.4 24.5
CH4 Fig. 4. Relative contributions to the annual GHG emissions. (a) CH4 vs. N2O; (b) landfill reservoirs vs. leachate systems.
GHG flux units are mg CH4 m−2 h−1 for CH4 and μg N2O m−2 h−1 for N2O. SD: Standard Deviation.
24
X. Wang et al. / Science of the Total Environment 596–597 (2017) 18–25
N 2O emissions from the landfill reservoirs into consideration, the N 2O contributed further as much as 80.7% and 44.8% of the total emissions for DB and DF landfills, respectively (Fig. 4). The results clearly demonstrated that the N2O emissions from the leachate treatment of sanitary landfills were significant and could not be negligible. Nowadays, the investigation of bacterial community structure in leachate systems are in progress, to find out the key differences with that of municipal wastewater treatment plants leading to the significant N2O emissions. Similar to previous reports that N2O emissions mainly occurred as aerobic processes in A/O-ultrafiltration-nanofiltration and A/Oultrafiltration wastewater treatment systems (Kong et al., 2016; Wang H.Q. et al., 2016; Wang E. et al., 2016), nitrifying tanks were the main sources of N2O emissions and denitrifying tank made a small contribution (Table 3). The average N2O emission factor (EF) of removed N from leachates was 8.9%, 11.9% and 10.2% for DB, DF and NJ landfills, respectively, far above that of municipal wastewater treatment plants (0.5%) (IPCC, 2006) but within the range of 0–25% reported previously (Bollon et al., 2016; Law et al., 2012). In this study, N2O EF from leachate was mainly affected by pH, aeration rate, oxidation-reduction potential and degradable organic carbon, which were fully discussed in the previous study (Wang et al., 2014). While the discrepancies of N2O emission between leachate treatment and municipal wastewater treatment are probably attributed to the water characteristics of leachate with high NH+ 4 -N content (Desloover et al., 2012), high salinity (Liu et al., 2015) and relatively low organic carbon (Pan et al., 2013). The key factor leading significant N2O emission from leachate is being studied in our lab. There are opportunities to enhance N removal efficiencies and reduce N2O emission factors in DB and DF landfills by controlling operation conditions. Further reducing N contents in effluents of DB and DF landfills is necessary to minimize the environmental impacts of discharged leachates. However, total N2O emissions from leachate treatment could increase if reduced EF is not traded off by increased N removal. DF landfill was closed in 2009 and the COD was already much lower than that in the ongoing DB landfill (Table 2). But the NH+ 4 content in DF leachate was still comparable to that in DB landfill and significant N2O emission occurred in the leachate treatment. These results suggest that a substantial amount of N2O is emitted from either the active landfills or the closed ones. From the view of the whole period of landfilling, intense N2O emissions could last a long time. As for Xiamen City, there were two leachate systems treating the fresh leachate in DB and the aged leachate in DF. Combined with the annual N2O emission of the systems and Xiamen's population of 3.67 million, the N2O emission from the landfill leachate treatment was estimated to be 8.55 g N2O-N capita− 1 yr− 1. This is higher than 3.2 g N2O-N capita−1 yr−1 from municipal wastewater treatment plants assumed by IPCC (2006). This estimate further demonstrates the importance of N2O emissions from landfills, particularly from leachate treatment systems. As for NJ landfill, even though N2O emissions from leachate were taken into account, the N2O only contributed 4.6% of the total emission from NJ landfill's reservoir and leachate (Fig. 4b). This was because only a small proportion of N in the influent was removed (N removal rate = 14.9%) by the current operation and management of the leachate system. Considering the higher EF achieved in NJ landfill, N2O emissions from leachate treatments would increase if N contents in effluents were further reduced. Therefore managed landfills, which are widely distributed throughout the suburbs of county cities in China, could become the potential significant sources of N2O emission. 5. Conclusions As for DB, DF and NJ landfill reservoirs, CH4 was the main greenhouse gas, which contributed 76.2%, 98.4% and 96.8% of the total emissions from the reservoir in CO2-eq, respectively. Management improvement, collection and use of LFGs can significantly reduce CH4
emissions from sanitary landfills. Moreover, a significant amount of N2O emissions occurred in the leachate treatments of sanitary landfills, where N2O emissions accounted for 76.2% and 44.3% of the annual GHG in the whole DB and DF landfills. The EF of the leachate treatment systems was in the range of 8.9–11.9% of the removed nitrogen. The N2O emissions would be even more than CH4 emissions in term of CO2-eq. Therefore, not only CH4 emissions but also N2O emissions should be included the inventory of greenhouse gas emissions from landfills. Acknowledgments This research was financially supported by the ‘Strategic Priority Research Program-Climate Change: Carbon Budget and Relevant Issues’ of the Chinese Academy of Sciences (Grant No. XDA05020602), and the National Natural Science Foundation of China (Grant No. 41475130). References Bae, J.H., Kim, S.K., Chang, H.S., 1997. Treatment of landfill leachates: ammonia removal via nitrification and denitrification and further COD reduction via Fenton's treatment followed by activated sludge. Water Sci. Technol. 366, 341–348. Bollon, J., Filali, A., Fayolle, Y., Guerin, S., Rocher, V., Gillot, S., 2016. N2O emissions from full-scale nitrifying biofilters. Water Res. 102, 41–51. Bremner, J.M., Blackmer, A.M., 1978. Nitrous oxide: emission from soils during nitrification of fertilizer nitrogen. Science 199, 295–296. Cai, Z.C., 2012. Greenhouse gas budget for terrestrial ecosystems in China. Sci. China Earth Sci. 55, 173–182. Chinese NEPA, 2002. Water and Wastewater Monitoring Methods. fourth ed. Chinses Environmental Science Publishing House, Beijing, China. Desloover, J., Vlaeminck, S.E., Clauwaert, P., Verstraete, W., Boon, N., 2012. Strategies to mitigate N2O emissions from biological nitrogen removal systems. Curr. Opin. Biotechnol. 23, 474–482. Harborth, P., Fuß, R., Münnich, K., Flessa, H., Fricke, K., 2013. Spatial variability of nitrous oxide and methane emissions from an MBT landfill in operation: strong N2O hotspots at the working face. Waste Manag. 33, 2099–2107. Hu, Q.W., Cai, J.Y., Yao, B., Wu, Q., Wang, Y.Q., Xu, X.L., 2015. Plant-mediated methane and nitrous oxide fluxes from a carex meadow in Poyang Lake during drawdown periods. Plant Soil 400, 1–14. IPCC, 2006. Wastewater treatment and discharge. In: Eggleston, H.S., Buendia, L., Miwa, K., Ngara, T., Tanabe, K. (Eds.), 2006 IPCC Guidelines for National Greenhouse Gas Inventories Prepared by the National Greenhouse Gas Inventories Programme. vol. 5. IGES, Japan. IPCC, 2007. Waste management in climate change 2007: mitigation. In: Metz, B., Davidson, O.R., Bosch, P.R., Dave, R., Meyer, L.A. (Eds.), Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge. IPCC, 2013. Climate change 2013: the physical science basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge. Kong, Q., Wang, Z.B., Niu, P.F., Miao, M.S., 2016. Greenhouse gas emission and microbial community dynamics during simultaneous nitrification and denitrification process. Bioresour. Technol. 210, 94–100. Law, Y.Y., Ni, B.J., Lant, P., Yuan, Z.G., 2012. N2O production rate of an enriched ammoniaoxidising bacteria culture exponentially correlates to its ammonia oxidation rate. Water Res. 46, 3409–3419. Lin, L., Lan, C.Y., Huang, L.N., Chan, G.Y.S., 2008. Anthropogenic N2O production from landfill leachate treatment. J. Environ. Manag. 87, 341–349. Liu, M., Yang, Q., Peng, Y.Z., Liu, T.T., Xiao, H., Wang, S.Y., 2015. Treatment performance and N2O emission in the UASB-A/O shortcut biological nitrogen removal system for landfill leachate at different salinity. J. Ind. Eng. Chem. 32, 63–71. Maag, M., Vinther, F.P., 1996. Nitrous oxide emission by nitrification and denitrification in different soil types and at different soil moisture contents and temperatures. Appl. Soil Ecol. 4, 5–14. Ma, Z.Y., Gao, Q.X., 2011. Guideline for Calculating GHG Emissions of Waste Disposal. Science Press, Beijing, China, p. 86. Monni, S., Pipatti, R., Lehtilä, A., Savolainen, I., Syri, S., 2006. Global Climate Change Mitigation Scenarios for Solid Waste Management. VTT Technical Research Centre of Finland. National Development and Reform Committee of the People's Republic of China, 2013. Second National Communication on Climate Change of the People's Republic of China. Pan, Y.T., Ni, B.J., Bond, P.L., Ye, L., Yuan, Z.G., 2013. Electron competition among nitrogen oxides reduction during methanol-utilizing denitrification in wastewater treatment. Water Res. 47, 3273–3281. Qiu, Y., Shi, H.C., He, M., 2010. Nitrogen and phosphorous removal in municipal wastewater treatment plants in China: a review. Int. J. Chem. Eng. 2010, 1–10. Renou, S., Givaudan, J.G., Poulain, S., Dirassouyan, F., Moulin, P., 2008. Landfill leachate treatment: review and opportunity. J. Hazard. Mater. 150, 468–493. Rinne, J., Pihlatie, M., Lohila, A., Thum, T., Aurela, M., Tuovinen, J.P., Laurila, T., Vesala, T., 2005. Nitrous oxide emissions from a municipal landfill. Environ. Sci. Technol. 39, 7790–7793. Song, C.C., Wang, Y.S., Wang, Y.Y., Zhao, Z.C., 2006. Emission of CO2, CH4 and N2O from freshwater marsh during freeze-thaw period in Northeast of China. Atmos. Environ. 40, 6879–6885.
X. Wang et al. / Science of the Total Environment 596–597 (2017) 18–25 Sutka, R.L., Ostrom, N.E., Ostrom, P.H., Breznak, J.A., Gandhi, H., Pitt, A.J., Li, F., 2006. Distinguishing nitrous oxide production from nitrification and denitrification on the basis of isotopomer abundances. Appl. Environ. Microbiol. 72, 638–644. UK Environment Agency, 2010. Guidance on Monitoring Landfill Gas Surface Emissions. US EPA, 2009. Inventory of US greenhouse gas emissions and sinks: 1990–2009. http:// www.epa.gov/climatechange/emissions/usinventoryreport.html. Wang, E., Xing, L.Z., Wu, G.X., Guan, Y.T., 2016. Nitrous oxide emission during nitrification of influents with different ammonium concentrations. Environ. Eng. Manag. J. 15, 19–25. Wang, H.Q., Guan, Y.T., Min, P., Wu, G.X., 2016. Aerobic N2O emission for activated sludge acclimated under different aeration rates in the multiple anoxic and aerobic process. J. Environ. Sci. 43, 70–79.
25
Wang, X.J., Jia, M.S., Chen, X.H., Xu, Y., Lin, X.Y., Kao, C.M., Chen, S.H., 2014. Greenhouse gas emissions from landfill leachate treatment plants: a comparison of young and aged landfill. Waste Manag. 34, 1156–1164. Wrage, N., Velthof, G.L., Beusichem, M.L.V., Oenema, O., 2001. Role of nitrifier denitrification in the production of nitrous oxide. Soil Biol. Biochem. 33, 1723–1732. Wu, D., Wang, C., Dolfing, J., Xie, B., 2015. Short tests to couple N2O emission mitigation and nitrogen removal strategies for landfill leachate recirculation. Sci. Total Environ. 512–513, 19–25. Zhou, H., Meng, A.H., Long, Y.Q., Li, Q.H., Zhang, Y.G., 2014. An overview of characteristics of municipal solid waste fuel in China: physical, chemical composition and heating value. Renew. Sust. Energ. Rev. 36, 107–122.
Contents lists available at ScienceDirect
Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv
Leachate treatment in landfills is a significant N2O source Xiaojun Wang a, Mingsheng Jia a, Chengliang Zhang a, Shaohua Chen a,⁎, Zucong Cai b,⁎⁎ a b
CAS Key Laboratory of Urban Pollutant Conversion, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, China School of Geography Sciences, Nanjing Normal University, Nanjing 210023, China
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
• N2O emissions from leachate system were intensive, while often ignored. • The static chamber method was adopted to measure GHG emissions in landfills. • A first set of GHG emissions from reservoirs and leachate in landfills was collected. • High N2O EF of 8.9–11.9% was achieved in leachate treatment systems. • N2O should be included in the inventory of GHG emissions from landfills.
a r t i c l e
i n f o
Article history: Received 9 February 2017 Received in revised form 1 April 2017 Accepted 5 April 2017 Available online xxxx Editor: Jay Gan Keywords: N2O emissions Leachate treatment systems Landfill reservoirs GHG inventory IPCC
a b s t r a c t The importance of methane (CH4) emissions from landfills has been extensively documented, while the nitrous oxide (N2O) emissions from landfills are considered negligible. In this study, three landfills were selected to measure CH4 and N2O emissions using the static chamber method. Dongbu (DB) and Dongfu (DF) landfills, both located in Xiamen city, Fujian Province, were classified as sanitary. The former started to receive solid waste from Xiamen city in 2009, and the latter was closed in 2009. Nanjing (NJ) landfill, located in Nanjing county, Fujian Province, was classified as managed. Results showed that for the landfill reservoirs, CH4 emissions were significant, while N2O emissions occurred mainly in operating areas (on average, 16.3 and 19.0 mg N2O m−2 h−1 for DB and NJ landfills, respectively) and made a negligible contribution to the total greenhouse gas emissions in term of CO2 equivalent. However, significant N2O emissions were observed in the leachate treatment systems of sanitary landfills and contributed 72.8% and 45.6% of total emissions in term of CO2 equivalent in DB and DF landfills, respectively. The N2O emission factor (EF) of the leachate treatment systems was in the range of 8.9–11.9% of the removed nitrogen. The total N2O emissions from the leachate treatment systems of landfills in Xiamen city were estimated to be as high as 8.55 g N2O-N capita−1 yr−1. These results indicated that N2O emissions from leachate treatment systems of sanitary landfills were not negligible and should be included in national and/or local inventories of greenhouse gas emissions. © 2017 Elsevier B.V. All rights reserved.
1. Introduction ⁎ Correspondence to: S. Chen, CAS Key Laboratory of Urban Pollutant Conversion, Institute of Urban Environment, Chinese Academy of Sciences, 1799 Jimei Road, Xiamen 361021, China. ⁎⁎ Corresponding author. E-mail addresses: [email protected] (X. Wang), [email protected] (M. Jia), [email protected] (C. Zhang), [email protected] (S. Chen), [email protected] (Z. Cai).
http://dx.doi.org/10.1016/j.scitotenv.2017.04.029 0048-9697/© 2017 Elsevier B.V. All rights reserved.
Waste disposal sites (solid waste and wastewater) are recognized as important sources of global anthropogenic greenhouse gas (GHG) and contributed approximately 3–4% of the annual budget in term of their CO2 equivalent (IPCC, 2007). In China, the GHG emissions from waste
X. Wang et al. / Science of the Total Environment 596–597 (2017) 18–25
treatment were estimated to account for 1.5% of the total national anthropogenic GHG emissions in 2005 (National Development and Reform Committee of China, 2013). Landfill is one of the main disposal methods of solid wastes around the world, and CH4 emissions from landfills are taken into account for national inventories of GHG emissions. In the United States, the CH4 emission from landfills was estimated to account for approximately 2.4% of the national anthropogenic GHG emissions and up to 21.6% of the total anthropogenic CH4 emissions between 1990 and 2008 (US EPA, 2009). The CH4 emissions from solid waste disposal are predicted to continuously increase as economic development advances and population rise (Monni et al., 2006). On the other hand, due to the strict anaerobic conditions inside landfills, nitrification of ammonium, which takes place under aerobic conditions (Bremner & Blackmer, 1978), is inhibited. And denitrification of nitrate, which occurs under anaerobic conditions (Wrage et al., 2001), is processed completely. Thus, as a byproduct of nitrification and an intermediate product of denitrification processes (Maag & Vinther, 1996; Sutka et al., 2006), N2O emissions from landfills are assumed to be negligible and are not taken into account in the 2007 IPCC Guidelines for National Greenhouse Gas Inventories (IPCC, 2007). However, the assumption that N2O emissions from landfills are negligible is not accurate. In situ measurements of N2O emissions from landfills showed that their N2O fluxes were one or more orders of magnitudes higher than those from forestlands, grasslands and farmlands (Cai, 2012; Rinne et al., 2005). There are several possible hotspots of N2O production and emission in landfills. One of the possible hotspots is the operating area where both aerobic and anaerobic conditions co-exist. Here, the produced N2O can be directly emitted into the atmosphere because no coverage prevents its transport. It has been reported that in a case-study of landfills, the peak N2O flux was up to 200 g CO2-eq m−2 h−1 (Harborth et al., 2013). Another possible hotspot is the landfill leachate treatment system. In China, landfills are required to establish an integrated leachate treatment system, and the effluent of leachate must reach a standard before discharged into water bodies in accordance with the Chinese National Standard (GB16889-2008). Nitrogen in leachates is generally removed by processes of alternative biological nitrification and denitrification (Qiu et al., 2010). On account of high contents of ammonium and organic matter (Bae et al., 1997; Renou et al., 2008), the N removal processes of landfill leachates are assumed to be active in N2O emission (Desloover et al., 2012). However, GHG emissions of the wastewater section consist only of municipal and industrial wastewater collection and treatment, not landfill leachates (IPCC, 2007). In the present study, comprehensive measurements of CH4 and N2O emissions from landfill reservoirs and leachate treatments were conducted in three landfills differing in classification and status. The objectives of the study were to identify the hotspots of CH4 and N2O emissions and to evaluate the contribution of N2O emission to total CH4 and N2O emissions from landfills in term of global warming potential. 2. Materials and methods 2.1. Landfill description Landfills are classified into three grades according to the environmental protection measures in China, i.e., simple landfills, managed landfills and sanitary landfills (Ma & Gao, 2011). For simple landfills, there are no environmental protection measures preventing leachates from infiltrating underground water, or landfill gases (LFGs) from being emitted into the atmosphere. Simple landfills are mainly distributed throughout countryside. Environmental protection measures are available for managed landfills. These measures prevent leachates from infiltrating underground water and require that leachates are treated, but landfills are not covered well by plastic film and/or soil, and the facilities for LFGs collection are not available. Managed landfills
19
are primarily distributed throughout the suburbs of county cities. Sanitary landfills have perfect environmental protection measures and facilities for preventing leachates from infiltrating underground water and halting LFG emission into the atmosphere by treating leachates and collecting and utilizing LFGs. Except for in the operating area, the reservoirs of landfills are well covered by high density polyethylene (HDPE) film. Sanitary landfills are mainly distributed in the suburbs of middle and large cities. For this study, three landfills with different management levels in Fujian Province were selected. CH4 and N2O emissions were sampled at these landfills over a one-year timeframe. (1) Dongbu Landfill (DB) in Xiamen City: classified as an active sanitary landfill, DB is designed for waste loading of 7.29 million tons and a 30-year expected operating period. From 2009 to now, DB landfill received municipal solid waste at an average load of 2100 tons per day. The reservoirs occupied 25.88 ha. Except for the operating area, landfill reservoirs are all covered by HDPE (Fig. 1a). The LFG collection system is established for CH4 recovery. The compositions of solid waste received by DB landfill are characterized and shown in Table 1. (2) Dongfu Landfill (DF) in Xiamen City: DF was also a sanitary landfill, but closed in 2009. The whole reservoir of landfill, occupying 18 ha, was covered by HDPE film, followed by a soil layer of approximately 0.5 m in depth with grass restoration (Fig. 1b). The landfilled waste reached a total of 3.8 million tons. The LFG collection system was also installed and collected LFG was used for electricity power generation. (3) Nanjing Landfill (NJ) in Nanjing County: NJ is an active managed landfill (Fig. 1c) and mainly receives the domestic solid waste from Nanjing County from 2008 to present, with a daily average load of 70 tons. The area of reservoir was 1.15 ha. 2.2. Leachate treatment systems in landfills Landfill leachates were first stored in leachate ponds and then treated by biological processes. A/O-ultrafiltration-nanofiltration, A/Oultrafiltration and oxidation ditches were applied to treat landfill leachates in DB, DF and NJ landfills, respectively (Fig. 2). The treating capacity and water characteristics of the influent and effluent were presented in Table 2. The N removal rates were, on average, 72.9% and 79.7% in DB and DF leachates, respectively. Since the oxidation ditch in NJ was poorly managed, the N removal rate was extremely low (14.5%). 2.3. Gas sampling For DB landfill reservoirs, six plots were set up in the operating area and six plots were set up in HDPE film covered areas to measure CH4 and N2O fluxes. CH4 and N2O fluxes were measured using the static chamber method in a two week intervals from April 2012 to April 2013. The CH4 and N2O emissions from DF landfill and NJ landfill reservoirs were measured by the static chamber method after creating grid cells of 40 × 40 m (Fig. 3a and b), following the ‘Guidance on monitoring landfill gas surface emissions’ proposed by the UK Environmental Agency (UK Environment Agency, 2010). For DF landfill reservoirs, CH4 and N2O fluxes from each cell were measured twice in January and July 2014, respectively. For NJ landfill reservoirs, CH4 and N2O fluxes from each cell were measured six times over a one-year timeframe (February 2015–March 2016) at an interval of two months. For the leachate storage and treatment systems, CH4 and N2O fluxes from each compartment were measured and gas sampling plots were shown in Fig. 2. Two methods, floating static chamber and gas flow rate, were employed to measure CH4 and N2O fluxes from a nonaerated surface and an agitated surface, respectively. The volumetric flow rate was obtained by the recorded flow velocity (4500 Pocket Weather Tracker, Kestrel) multiplying by the corresponding area of the off-gas vent. The mass flow rate was calculated from the measured
20
X. Wang et al. / Science of the Total Environment 596–597 (2017) 18–25
Fig. 1. A panorama of the case-study landfills in the suburb of Xiamen (a) the map of sampling sites; (b) DB: an active sanitary landfill; (c) DF: a closed sanitary landfill; (d) NJ: an active managed landfill.
gas concentration and the volumetric flow rate of the off-gas originating from the covered reactor zones. The procedure for sampling was described in detail by Wang et al. (2014). Three sampling rounds were conducted during April/May of 2013 for DB and DF leachate treatment systems. For NJ landfill leachate treatment system, the measurements of CH4 and N2O fluxes were undertaken every two months using the floating static chamber, synchronized with the measurements in landfill reservoirs.
landfill reservoirs was combined with spatial interpolation (Inverse Distance Weighting, IDW) based on geostatistics (ArcGIS 9.3, ESRI, USA). To compare the contributions of CH4 and N2O emissions to global warming potential, the emissions were expressed in CO2 equivalents (CO2-eq), in which CH4 and N2O had 25 and 298 times the global warming potential than that of CO2, respectively, at a time scale of 100 yr (IPCC, 2013). The N2O emission factor (EF) of removed N in landfill leachates was used to evaluate the capacity of N2O emission from the leachate system (Eq. (2)).
2.4. Analyses and calculation To measure CH4 and N2O fluxes by the static chamber method, five sequential gas samples were withdrawn from chamber headspace by syringe at 10-min intervals for landfill reservoirs and 5-min intervals for leachate treatment systems. Off-gas samples were also collected to determine CH4 and N2O fluxes from aerated bioreactors. All gas samples were analyzed by using a gas chromatograph (GC, Agilent 7890A, Palo Alto, CA), fitted with a flame ionization detector (FID) for CH4 detection and an electron capture detector (ECD) for N2O detection. The fluxes were calculated based on the change in concentrations over time (dC/ dt), chamber height and temperature correction (Eq. (1)) (Song et al., 2006). F¼
dC M 273 h dt V0 T þ 273
ð1Þ
where F is the flux (mg CH4 m−2 h−1 or mg N2O m−2 h−1); dC/dt is the change in CH4 or N2O concentration over time; M is the mole mass of CH4 or N2O; V0 is the mole volume of the target gas under standard conditions; T is the air temperature; h is the headspace height of the chamber. The annual CH4 and N2O emissions were calculated using the average fluxes multiplied by the time of year in hours and the size of the corresponding study sections. The average fluxes were assumed to be representative of annual emissions for each section. The calculation of annual emissions from the full-scale field monitoring in DF and NJ
EF ¼
E MN;Inf −MN;Eff
ð2Þ
where E is the N2O emission of whole treatment system, MN,Inf is the total amount of nitrogen influent (kg N d−1) and MN,Eff is the total amount of nitrogen effluent (kg N d−1). The denominator in the equation was the nitrogen amount removed by the treatment system. An elemental analyzer (Vario MAX, Elementar, Germany) was used to determine carbon and nitrogen contents in solid waste. For leachate, − − the ammonium (NH+ 4 ), nitrite (NO2 ), nitrate (NO3 ) and total nitrogen (TN) concentrations were analyzed by using spectrophotometry described in standard methods (Chinese NEPA, 2002). The colorimetric method was adopted to determine the chemical oxygen demand (COD) of landfill leachate with COD analyzer (5B-1, Lianhua Technology, China). 3. Results In the study, CH4 and N2O sources were divided into landfill reservoir and leachate categories. The former was further divided into operating area and covered area (except for the closed DF landfill, which had no operating area) and the latter was further divided into leachate storage pond and treatment system. The CH4 and N2O emissions from each compartment of DB, DF and NJ were presented as follows.
Table 1 Physical and chemical characteristics of municipal solid waste disposed in DB landfill. Compositiona %
MSW a
Food waste
Papers
Plastics
Textiles
Woods
Glasses
Metals
Dust
51.5
9.3
15.0
5.1
2
2.9
0.7
13.5
Provided by Environmental Sanitation Management Department of Xiamen City.
Moisture %
C%
N%
Food waste C%
N%
48.4 ± 2.5
45.3 ± 4.4
1.12 ± 0.04
49.5 ± 2.7
2.57 ± 0.26
X. Wang et al. / Science of the Total Environment 596–597 (2017) 18–25
21
Fig. 2. Sampling sections of leachate treatment processes at three landfills. (a) DB for the young leachate; (b) DF for the aged leachate; (c) NJ for the young leachate.
3.1. CH4 and N2O emissions from DB landfill HDPE film could prevent the CH4 and N2O emissions effectively in DB landfill. The CH4 fluxes from the HDPE film covered surface ranged between 0.179 and 70.3 mg CH 4 m − 2 h − 1 with an average of 16.3 mg CH4 m− 2 h− 1. N2O fluxes ranged between 0 and 156 μg N2O m−2 h−1 with an average of 33 μg N2O m−2 h−1. N2O emissions from the landfill reservoir mainly occurred in the operating area. The mean N2O flux reached 16.4 mg N2O m−2 h− 1 in the operating area, which contributed 97% of the total N2O emission from the reservoir. The CH4 and N2O emissions from the DB landfill reservoir were 96.0 t CH4 yr−1 and 2.52 t N2O yr−1, respectively (Table 3). N2O and CH4 contributed 23.8% and 76.2% of the total emissions from the reservoir in CO2-eq, respectively. A substantial amount of CH4 emission was detected from the leachate storage pond compared to the landfill reservoir. The landfill discharged leachate at approximately 470 m3 per day (Table 2) and the CH4 flux from the storage pond was as high as 18.0 g CH4 m−2 h−1 on average. The accumulated CH4 emission from the storage pond was as large as 4360 t CH4 yr− 1 (Table 3). A small amount of CH4 emission occurred in leachate treatment systems as well, particularly from the denitrifying tank. The annual CH4 emission from the leachate treatment system was 2.26 t CH4 yr−1. In sharp contrast with CH4 emission from the leachate, N2O emissions mainly occurred in the leachate treatment system (34.42 t N2O yr−1) and the emission from the leachate storage pond was relatively small (87.8 kg N2O yr−1). In the leachate system, nitrifying processes were the dominant N2O source, accounting for 74.7% and 16.9% of the total
N2O emission for nitrifying tanks 1 and 2, respectively. The denitrifying tank contributed only 8.2% of the total N2O emission from the leachate treatment system (Table 3). Considering that the CH4 emitted from leachate storage pond was used for electricity generation, it was not included as an emission to the atmosphere. Therefore, the total CH4 and N2O emissions to the atmosphere from the whole DB landfill was 13.5 Gg CO2-eq yr−1, in which 18.2% were contributed by CH4 and 81.8% by N2O (Fig. 4a). Of these emissions, 23.4% were from the reservoir and 76.6% were from the leachate (Fig. 4b). 3.2. CH4 and N2O emissions from DF landfill DF landfill was closed in 2009 and whole reservoir was covered by HDPE film, followed by a soil layer and grasses that were grown on the covered soils. The CH4 and N2O emissions at the 98 sampling sites of the grid cells were detectable. Both CH4 and N2O fluxes had high spatial variability with the range of coefficient of variation (CV) from 268% to 421%. The averaged fluxes of CH4 and N2O were 139 mg CH4 m−2 h−1 and 189 μg N2O m−2 h−1, respectively (Table 4). There were no significant differences between the fluxes measured in summer and winter in terms of CH4 and N2O (P N 0.05). The annual CH4 and N2O emissions from DF landfill reservoir were 219.6 t CH4 yr−1 and 0.3 t N2O yr−1 (Table 3), respectively, and the contribution of N2O to the total emission was only 1.6% in terms of CO2-eq. DF landfill discharged leachate at approximately 250 m3 d− 1 (Table 2). The CH4 fluxes from the leachate storage pond were much lower than those of DB landfill. The accumulated CH4 emission in DF
674.1 ± 277.6 970.1 ± 408.8 2120.0 ± 270.1
leachate storage pond was only 0.05 t CH4 yr−1 (Table 3). The CH4 from the leachate storage pond of DF landfill was not collected, but emitted to the atmosphere directly. Thus, the emission contributed to the total emission from the DF landfill. Like DB landfill, the leachate treatment system of DF landfill was also a big source of N2O, mainly from nitrifying tanks 1 and 2. The total N2O emission from the leachate treatment system was 14.94 t N2O yr−1, of which 68.1% was contributed by nitrifying tank 1 and 20.2% by nitrifying tank 2 (Table 3). The total CH4 and N2O emissions to the atmosphere from DF landfill were 10.0 Gg CO2-eq yr−1. The contribution of CH4 (54.8%) was slightly more than that of N2O (45.2%) to the total emission (Fig. 4a). The landfill reservoir and leachate contributed 55.6% and 44.4%, respectively (Fig. 4b).
30.7 ± 29.8 2068.8 ± 128.2 2127.2 ± 277.8 1491.4 ± 107.2 Fresh Aged Fresh 470 250 15 DB DF NJ
3.3. CH4 and N2O emissions from NJ landfill
“-” means non-detectable.
COD TN
663.1 ± 129.6 438.1 ± 112.9 1618.4 ± 110.0 608.5 ± 292.3 384.7 ± 146.7 168.2 ± 103.3 12.0 ± 11.4 10.9 ± 5.4 1173.3 ± 44.2
COD
8295 ± 1090 2150 ± 77 2776.5 ± 125.2
TN
Fig. 3. The designed sampling sites of full-scale field monitoring campaigns in (a) closed DF sanitary landfill and (b) active NJ county landfill.
16.9 ± 1.7 14.1 ± 11.3 276.7 ± 19.8
2448.5 ± 265.9 2160.5 ± 222.0 1875.3 ± 43.0
1.5 ± 2.1 6.8 ± 8.7 217.2 ± 104.1
NO− 3 -N NO− 2 -N NH+ 4 -N
Effluent (mg L−1)
NO− 3 -N NO− 2 -N NH+ 4 -N
Influent (mg L−1)
Leachate status Loading (m3 d−1)
Table 2 Water characteristics of leachate treatment systems in three landfills.
72.9% 79.7% 14.5%
X. Wang et al. / Science of the Total Environment 596–597 (2017) 18–25 TN removal rate
22
The reservoir of NJ landfill was covered by a soil layer, except for the operating area. The averaged CH4 fluxes over each measurement campaign ranged from 405 to 1914 mg CH4 m−2 h−1 with an overall average of 1089 ± 517 mg CH4 m− 2 h− 1. The corresponding N2O fluxes ranged from 1.33 to 6.57 mg N2O m−2 h−1 with an overall average of 2.95 ± 1.43 mg N2O m− 2 h− 1. The annual CH4 and N2O emissions from the reservoir were 122 t CH4 yr−1 and 0.34 t N2O yr−1, respectively. N2O contributed only 3.2% of the total emission in term of CO2-eq. The emissions from leachate were 0.16 t CH4 yr−1 and 0.15 t N2O yr− 1 (Table 3). The CH4 produced in the leachate storage pond was not collected and used in NJ landfill. Therefore, the total
X. Wang et al. / Science of the Total Environment 596–597 (2017) 18–25 Table 3 Annual CH4 and N2O emissions from landfill reservoirs and individual bioreactor compartments of three leachate treatment systems. Source
Reservoir Leachate Storage pond Treatment system Denitrifying tank Nitrifying tank-1 Nitrifying tank-2 Oxidation ditch Secondary sedimentation tank Sludge thickening tank Nanofiltration concentrate tank
CH4 (t CH4 yr−1)
N2O (t N2O yr−1)
DB
DF
NJ
DB
DF
NJ
96.0 2.26a 4360 2.26 1.37 0.55 0.09 – – 0.14 0.11
219.6 0.33 0.05 0.28 0.05 0.08 0.07 – – 0.08 –
122 0.16 0.14 0.02 – – – 0.02 b0.01 – –
2.52 34.50 0.09 34.41 2.83 25.72 5.82 – – 0.04 b0.01
0.3 14.94 b0.01 14.94 1.74 10.17 3.02 – – 0.01 –
0.34 0.15 0.01 0.14 – – – 0.14 b0.01 – –
a The CH4 emitted from the leachate storage pond was used for electricity generation, so it was not included into the GHG inventory of DB landfill.
emission to the atmosphere, including the CH4 emissions from leachate, was 3.2 Gg CO2-eq yr−1. Unlike the DB and DF landfills, the contribution of leachate to the total emission was only 1.5% (Fig. 4a) and N2O emissions from the reservoir and leachate accounted for only 4.6% of the total (Fig. 4b).
23
4. Discussions 4.1. Negligible N2O emission from landfill reservoirs N2O emissions from landfills are not included in the 2007 IPCC Guidelines for National Inventory of Greenhouse Gas Emissions because the emissions are thought to be negligible. This is true if only the emissions from landfill reservoirs are considered. HDPE film coverage of sanitary landfills effectively prevented N2O emissions from landfill reservoirs. The mean N2O flux from the HDPE film covered areas of DB landfill reservoir (0.033 mg N2O m− 2 h− 1) was much less than that from the operating area (16.3 mg N2O m−2 h−1) and the overall average from the NJ landfill reservoir (2.95 mg N2O m−2 h−1). The reservoir of DF landfill was covered by HDPE film, followed by a soil layer with grass growth (Fig. 1b). The N2O fluxes from the reservoir of DF landfill (averagely 188.6 μg N2O m−2 h−1, Table 4) were substantially larger than that from the HDPE surface of DB landfill, and slightly larger than those from subtropical terrestrial ecosystems (Hu et al., 2015). This indicates that the emitted N2O from the reservoir of DF landfill is likely produced mainly in the covered soil layer, rather than in the deposited wastes. The N2O emissions accounted for only 23.8%, 1.6% and 3.2% of the total CO2-eq emissions from DB, DF and NJ landfill reservoirs, respectively. For DB landfill reservoir, 97% of N2O emissions occurred in the operating area. 4.2. Significant N2O emissions from leachate treatment of sanitary landfills Landfill leachate was characterized by extremely high NH+ 4 concentrations, which were N2000 mg N L−1 in DB and DF landfills on average (Table 2). High N content in landfill leachate was attributed to the large proportion (28–60%) of food in municipal solid wastes in China. Compared with other components of solid waste, food waste had a higher N content leading to a low C/N ratio (Zhou et al., 2014). The large amount (55.1%) of food waste made the overall N content in Xiamen municipal solid wastes relatively high (Table 1). Several treatment processes have been developed to remove N in landfill leachates. A/Oultrafiltration-nanofiltration and A/O-ultrafiltration were applied to treat landfill leachates in DB and DF landfills, respectively (Fig. 2). The N removal efficiencies were 72.9% in DB and 79.7% in DF. These are comparable to those of lab-scale and full-scale treatment systems reported in the literatures (Lin et al., 2008; Wu et al., 2015), but N contents in effluents were still very high (Table 2). NH+ 4 content in the leachates of NJ landfill (1491 mg N L−1) was lower than those in DB and DF landfills. An oxidation ditch was used to remove N in the leachates in NJ landfill, but the efficiency was very low (14.5%). Further, the total N content in the effluent was even higher than those in DB and DF landfills (Table 2), indicating that the treatment system had to be improved to minimize the environmental impacts of discharged landfill leachates. Large N2O emissions from the leachate treatment systems were observed in DB and DF landfills. The N2O emissions from the leachate treatment system accounted for 76.2% and 44.4% of the summed emissions from landfill reservoirs and leachate treatment systems in term of CO 2-eq for DB and DF landfills, respectively. Taking the Table 4 Summary of CH4 and N2O fluxes from DF landfill reservoir, which was measured by following the ‘Guidance on monitoring landfill gas surface emissions’ proposed by the UK Environment Agency (2010). Target gas
Sampling time
Mean
Min
Max
SD
CV (%)
Percentage of negative value
N2 O
2014-01 2014-07 2014-01 2014-07
172.9 204.3 94.9 113.0
−47.1 −47.1 −6.3 −14.6
4211 6789 1555 1823
628.6 848.6 254.2 325.0
377.9 420.5 267.8 288.7
9.2 8.2 21.4 24.5
CH4 Fig. 4. Relative contributions to the annual GHG emissions. (a) CH4 vs. N2O; (b) landfill reservoirs vs. leachate systems.
GHG flux units are mg CH4 m−2 h−1 for CH4 and μg N2O m−2 h−1 for N2O. SD: Standard Deviation.
24
X. Wang et al. / Science of the Total Environment 596–597 (2017) 18–25
N 2O emissions from the landfill reservoirs into consideration, the N 2O contributed further as much as 80.7% and 44.8% of the total emissions for DB and DF landfills, respectively (Fig. 4). The results clearly demonstrated that the N2O emissions from the leachate treatment of sanitary landfills were significant and could not be negligible. Nowadays, the investigation of bacterial community structure in leachate systems are in progress, to find out the key differences with that of municipal wastewater treatment plants leading to the significant N2O emissions. Similar to previous reports that N2O emissions mainly occurred as aerobic processes in A/O-ultrafiltration-nanofiltration and A/Oultrafiltration wastewater treatment systems (Kong et al., 2016; Wang H.Q. et al., 2016; Wang E. et al., 2016), nitrifying tanks were the main sources of N2O emissions and denitrifying tank made a small contribution (Table 3). The average N2O emission factor (EF) of removed N from leachates was 8.9%, 11.9% and 10.2% for DB, DF and NJ landfills, respectively, far above that of municipal wastewater treatment plants (0.5%) (IPCC, 2006) but within the range of 0–25% reported previously (Bollon et al., 2016; Law et al., 2012). In this study, N2O EF from leachate was mainly affected by pH, aeration rate, oxidation-reduction potential and degradable organic carbon, which were fully discussed in the previous study (Wang et al., 2014). While the discrepancies of N2O emission between leachate treatment and municipal wastewater treatment are probably attributed to the water characteristics of leachate with high NH+ 4 -N content (Desloover et al., 2012), high salinity (Liu et al., 2015) and relatively low organic carbon (Pan et al., 2013). The key factor leading significant N2O emission from leachate is being studied in our lab. There are opportunities to enhance N removal efficiencies and reduce N2O emission factors in DB and DF landfills by controlling operation conditions. Further reducing N contents in effluents of DB and DF landfills is necessary to minimize the environmental impacts of discharged leachates. However, total N2O emissions from leachate treatment could increase if reduced EF is not traded off by increased N removal. DF landfill was closed in 2009 and the COD was already much lower than that in the ongoing DB landfill (Table 2). But the NH+ 4 content in DF leachate was still comparable to that in DB landfill and significant N2O emission occurred in the leachate treatment. These results suggest that a substantial amount of N2O is emitted from either the active landfills or the closed ones. From the view of the whole period of landfilling, intense N2O emissions could last a long time. As for Xiamen City, there were two leachate systems treating the fresh leachate in DB and the aged leachate in DF. Combined with the annual N2O emission of the systems and Xiamen's population of 3.67 million, the N2O emission from the landfill leachate treatment was estimated to be 8.55 g N2O-N capita− 1 yr− 1. This is higher than 3.2 g N2O-N capita−1 yr−1 from municipal wastewater treatment plants assumed by IPCC (2006). This estimate further demonstrates the importance of N2O emissions from landfills, particularly from leachate treatment systems. As for NJ landfill, even though N2O emissions from leachate were taken into account, the N2O only contributed 4.6% of the total emission from NJ landfill's reservoir and leachate (Fig. 4b). This was because only a small proportion of N in the influent was removed (N removal rate = 14.9%) by the current operation and management of the leachate system. Considering the higher EF achieved in NJ landfill, N2O emissions from leachate treatments would increase if N contents in effluents were further reduced. Therefore managed landfills, which are widely distributed throughout the suburbs of county cities in China, could become the potential significant sources of N2O emission. 5. Conclusions As for DB, DF and NJ landfill reservoirs, CH4 was the main greenhouse gas, which contributed 76.2%, 98.4% and 96.8% of the total emissions from the reservoir in CO2-eq, respectively. Management improvement, collection and use of LFGs can significantly reduce CH4
emissions from sanitary landfills. Moreover, a significant amount of N2O emissions occurred in the leachate treatments of sanitary landfills, where N2O emissions accounted for 76.2% and 44.3% of the annual GHG in the whole DB and DF landfills. The EF of the leachate treatment systems was in the range of 8.9–11.9% of the removed nitrogen. The N2O emissions would be even more than CH4 emissions in term of CO2-eq. Therefore, not only CH4 emissions but also N2O emissions should be included the inventory of greenhouse gas emissions from landfills. Acknowledgments This research was financially supported by the ‘Strategic Priority Research Program-Climate Change: Carbon Budget and Relevant Issues’ of the Chinese Academy of Sciences (Grant No. XDA05020602), and the National Natural Science Foundation of China (Grant No. 41475130). References Bae, J.H., Kim, S.K., Chang, H.S., 1997. Treatment of landfill leachates: ammonia removal via nitrification and denitrification and further COD reduction via Fenton's treatment followed by activated sludge. Water Sci. Technol. 366, 341–348. Bollon, J., Filali, A., Fayolle, Y., Guerin, S., Rocher, V., Gillot, S., 2016. N2O emissions from full-scale nitrifying biofilters. Water Res. 102, 41–51. Bremner, J.M., Blackmer, A.M., 1978. Nitrous oxide: emission from soils during nitrification of fertilizer nitrogen. Science 199, 295–296. Cai, Z.C., 2012. Greenhouse gas budget for terrestrial ecosystems in China. Sci. China Earth Sci. 55, 173–182. Chinese NEPA, 2002. Water and Wastewater Monitoring Methods. fourth ed. Chinses Environmental Science Publishing House, Beijing, China. Desloover, J., Vlaeminck, S.E., Clauwaert, P., Verstraete, W., Boon, N., 2012. Strategies to mitigate N2O emissions from biological nitrogen removal systems. Curr. Opin. Biotechnol. 23, 474–482. Harborth, P., Fuß, R., Münnich, K., Flessa, H., Fricke, K., 2013. Spatial variability of nitrous oxide and methane emissions from an MBT landfill in operation: strong N2O hotspots at the working face. Waste Manag. 33, 2099–2107. Hu, Q.W., Cai, J.Y., Yao, B., Wu, Q., Wang, Y.Q., Xu, X.L., 2015. Plant-mediated methane and nitrous oxide fluxes from a carex meadow in Poyang Lake during drawdown periods. Plant Soil 400, 1–14. IPCC, 2006. Wastewater treatment and discharge. In: Eggleston, H.S., Buendia, L., Miwa, K., Ngara, T., Tanabe, K. (Eds.), 2006 IPCC Guidelines for National Greenhouse Gas Inventories Prepared by the National Greenhouse Gas Inventories Programme. vol. 5. IGES, Japan. IPCC, 2007. Waste management in climate change 2007: mitigation. In: Metz, B., Davidson, O.R., Bosch, P.R., Dave, R., Meyer, L.A. (Eds.), Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge. IPCC, 2013. Climate change 2013: the physical science basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge. Kong, Q., Wang, Z.B., Niu, P.F., Miao, M.S., 2016. Greenhouse gas emission and microbial community dynamics during simultaneous nitrification and denitrification process. Bioresour. Technol. 210, 94–100. Law, Y.Y., Ni, B.J., Lant, P., Yuan, Z.G., 2012. N2O production rate of an enriched ammoniaoxidising bacteria culture exponentially correlates to its ammonia oxidation rate. Water Res. 46, 3409–3419. Lin, L., Lan, C.Y., Huang, L.N., Chan, G.Y.S., 2008. Anthropogenic N2O production from landfill leachate treatment. J. Environ. Manag. 87, 341–349. Liu, M., Yang, Q., Peng, Y.Z., Liu, T.T., Xiao, H., Wang, S.Y., 2015. Treatment performance and N2O emission in the UASB-A/O shortcut biological nitrogen removal system for landfill leachate at different salinity. J. Ind. Eng. Chem. 32, 63–71. Maag, M., Vinther, F.P., 1996. Nitrous oxide emission by nitrification and denitrification in different soil types and at different soil moisture contents and temperatures. Appl. Soil Ecol. 4, 5–14. Ma, Z.Y., Gao, Q.X., 2011. Guideline for Calculating GHG Emissions of Waste Disposal. Science Press, Beijing, China, p. 86. Monni, S., Pipatti, R., Lehtilä, A., Savolainen, I., Syri, S., 2006. Global Climate Change Mitigation Scenarios for Solid Waste Management. VTT Technical Research Centre of Finland. National Development and Reform Committee of the People's Republic of China, 2013. Second National Communication on Climate Change of the People's Republic of China. Pan, Y.T., Ni, B.J., Bond, P.L., Ye, L., Yuan, Z.G., 2013. Electron competition among nitrogen oxides reduction during methanol-utilizing denitrification in wastewater treatment. Water Res. 47, 3273–3281. Qiu, Y., Shi, H.C., He, M., 2010. Nitrogen and phosphorous removal in municipal wastewater treatment plants in China: a review. Int. J. Chem. Eng. 2010, 1–10. Renou, S., Givaudan, J.G., Poulain, S., Dirassouyan, F., Moulin, P., 2008. Landfill leachate treatment: review and opportunity. J. Hazard. Mater. 150, 468–493. Rinne, J., Pihlatie, M., Lohila, A., Thum, T., Aurela, M., Tuovinen, J.P., Laurila, T., Vesala, T., 2005. Nitrous oxide emissions from a municipal landfill. Environ. Sci. Technol. 39, 7790–7793. Song, C.C., Wang, Y.S., Wang, Y.Y., Zhao, Z.C., 2006. Emission of CO2, CH4 and N2O from freshwater marsh during freeze-thaw period in Northeast of China. Atmos. Environ. 40, 6879–6885.
X. Wang et al. / Science of the Total Environment 596–597 (2017) 18–25 Sutka, R.L., Ostrom, N.E., Ostrom, P.H., Breznak, J.A., Gandhi, H., Pitt, A.J., Li, F., 2006. Distinguishing nitrous oxide production from nitrification and denitrification on the basis of isotopomer abundances. Appl. Environ. Microbiol. 72, 638–644. UK Environment Agency, 2010. Guidance on Monitoring Landfill Gas Surface Emissions. US EPA, 2009. Inventory of US greenhouse gas emissions and sinks: 1990–2009. http:// www.epa.gov/climatechange/emissions/usinventoryreport.html. Wang, E., Xing, L.Z., Wu, G.X., Guan, Y.T., 2016. Nitrous oxide emission during nitrification of influents with different ammonium concentrations. Environ. Eng. Manag. J. 15, 19–25. Wang, H.Q., Guan, Y.T., Min, P., Wu, G.X., 2016. Aerobic N2O emission for activated sludge acclimated under different aeration rates in the multiple anoxic and aerobic process. J. Environ. Sci. 43, 70–79.
25
Wang, X.J., Jia, M.S., Chen, X.H., Xu, Y., Lin, X.Y., Kao, C.M., Chen, S.H., 2014. Greenhouse gas emissions from landfill leachate treatment plants: a comparison of young and aged landfill. Waste Manag. 34, 1156–1164. Wrage, N., Velthof, G.L., Beusichem, M.L.V., Oenema, O., 2001. Role of nitrifier denitrification in the production of nitrous oxide. Soil Biol. Biochem. 33, 1723–1732. Wu, D., Wang, C., Dolfing, J., Xie, B., 2015. Short tests to couple N2O emission mitigation and nitrogen removal strategies for landfill leachate recirculation. Sci. Total Environ. 512–513, 19–25. Zhou, H., Meng, A.H., Long, Y.Q., Li, Q.H., Zhang, Y.G., 2014. An overview of characteristics of municipal solid waste fuel in China: physical, chemical composition and heating value. Renew. Sust. Energ. Rev. 36, 107–122.